This invention concerns a method of forming or manufacturing a rack which has a longitudinal array of rack teeth and which is intended for engagement with a helically toothed pinion so that rotation of the pinion will effect in displacement of the rack member. These latter combinations are commonly known as rack and pinion gears and are well known in the art of vehicle steering systems.
The present invention is more particularly concerned with the formation or manufacture of a rack member of the type (hereinafter referred to as "of the type specified") having a longitudinal array of rack teeth which are intended for engagement with a helically toothed pinion having constant pitch helical teeth whereby rotation of the pinion will effect in displacement of the rack member along a longitudinal axis of that member, and in which at least some of the rack teeth are variable ratio teeth formed with varying pitch, with shapes that vary over the widthwise extent of the rack, and with tooth angles in the plane of the rack which are directed widthwise of said teeth and are inclined relative to the longitudinal axis and wherein said shapes and tooth angles vary between adjacent teeth so that when the variable ratio teeth are engaged by the pinion during displacement of the rack member they provide a relatively smooth variation in drive ratio between the pinion and rack member. A rack member of the type specified in combination with a conventional helical pinion is commonly known as a variable ratio rack and pinion gear and an example is to be found in U.K. Patent No. 1,356,172 which is primarily intended for vehicle steering.
In a conventional rack and pinion gear providing a constant ratio the rack teeth are of constant pitch, are of the same and constant shape over their widthwise extent and are parallel in their widthwise extent so that all of the teeth have the same tooth angle. Such a constant ratio rack member is conventionally formed by a simple broaching technique whereby a gang of cutters, which are identically shaped to each other and to correspond with the profile of the trough which is to be formed between each pair of adjacent teeth in the array, are displaced in parallel and along the tooth angle relative to a workpiece from which the rack member is to be formed to simultaneously cut the constant ratio rack teeth at the required pitch. However, with a variable ratio rack member of the type specified, both the tooth angle and the shape of the teeth will vary between two or more adjacent (variable ratio) teeth in the array. This variation may be for a part length only of the array of rack teeth so that teeth in one or more part lengths of the array may be intended to provide constant ratio when engaged by the pinion while teeth (variable ratio teeth) in one or more other parts of the array are intended to provide the variable ratio characteristic where the effective pitch changes as the pinion moves over the teeth in that part. Alternatively the teeth on the rack member may provide a constantly varying ratio whereby there is a slight ratio change during displacement of the pinion from any one tooth to another in the array on the rack member. Because of the change in the tooth angles, in the effective pitch and in the shapes (particularly as regards the variation in the profiles and the flank angles of the tooth flank surfaces) of the variable ratio teeth there is a problem in accurately forming or manufacturing these teeth (and thereby the array of rack teeth generally) in a manner which is economically and commercially viable and to the required accuracy for use, particularly in vehicle steering gears. The simple broaching technique with a gang of cutters as previously discussed is inappropriate, primarily because of the aforementioned changes in the tooth angles and in the profiles of the variable ratio teeth over their widthwise extent. However, prior proposals for forming the teeth include electro machining which is believed to be too expensive and too slow; coin pressing which requires considerable capital expense and is believed to be unacceptable for the reason that too much metal has to be displaced in the basic workpiece during the formation of the teeth, and a broaching/coining technique whereby an array of upstanding projections are rough formed on a workpiece by broaching followed by coining said projections to reduce their height and to shape them into rack teeth with the required characteristics, again with this general proposal it is believed that too much metal has to be displaced during the coining operation for the technique to be commercially acceptable. Discussions of the aforegoing proposals are to be found in the aforementioned U.K. Patent No. 1,356,172 and in U.K. Patent Specification No. 2,026,908. It is therefore an object of the present invention to provide a method of forming or manufacturing a rack member of the type specified which alleviates the disadvantages of the prior proposals.